Avalanche photodiode structures that have separate absorption and multiplication layers (SAM-APDs) can provide electrical output signals with high fidelity (i.e. low noise). For use in telecommunications applications, the APD is electrically biased such that the electrical response is substantially linear with optical power. Although the APD requires higher operating voltages, the internal gain of the APD provides a significant increase in receiver sensitivity compared to a PIN photodiode. This is important for realizing high speed optical receivers for high data rate communications networks.
A critical feature of APD design is to maintain gain uniformity across the active region of the device. The active region comprises a p-n junction formed by diffusion in the multiplication layer. However, the edge curvature of the diffusion profile causes locally elevated electric fields at the edge of the active region. The tendency for increase of electric fields at the edge of the active region is a basic property of the physics of finite-size planar p-n junctions. As seen in a prior art diode structure shown in FIG. 1, the electric field at the edge (E1 for single diffusion, and E1, E2 for double diffusion) is usually higher than that at center Ec due to the curvature effect.
The p-n junction is formed by opening a window, such as by photolithography, in a passivation layer above an intrinsic multiplication layer. Solid or gas sources are used to deliver a p-type dopant, such as zinc to the window, which is diffused under high heat into the crystal lattice of the multiplication layer. Because the flow of dopant molecules is both lateral and transverse a curved edge region is formed underneath the edges of the window. This curved portion of the p-n junction has a higher electric field than the planar portions in the center of the window.
These increased edge fields lead to larger gain at the edge of the active region which causes a poor gain uniformity and premature breakdown, typically referred to as “edge breakdown”. The poor gain uniformity has markedly deleterious effects on device performance, particularly the noise performance and bandwidth of the APD. For practical SAM-APDs, the breakdown-voltage uniformity across the entire active region should be within 5 to 10 percent, and preferably within one percent. The breakdown-voltage is the voltage at which the p-n junction is sufficiently reverse-biased to conduct a large current arising from a self-sustaining avalanche process, even in the absence of continuous optical power.
Reduction of the electric field intensity at the edge of the active region is a key for alleviating edge breakdown. One published technique for controlling edge breakdown in planar junctions is through controlling the diffused pn-junction profile (also called diffusion profile). Ultimately, the key to suppress the edge breakdown is to have a smooth transition profile at the edge of the active region.
As illustrated in FIG. 1, a p-n junction 10 is shaped to create a thicker multiplication layer with consequently lower electric fields at the edge 12 of the active region 14. The method for realizing such a shaped diffusion profile 10 is through the use of double diffusions of the same dopant (e.g. zinc) employing different diameter concentric windows for successive diffusions to different depths. The second diffusion edge can be smoothed out with the drive-in help of the dopant from the first diffusion region. Therefore, the curvature effect of the second diffusion edge 16 is alleviated. An example of this technique is disclosed in U.S. Pat. No. 6,515,315 by M. A. Itzler et al, assigned to the common owner of the present invention. The diffusion depth of the first and second diffusion steps should be very carefully optimized to achieve the highest electric field located in the center region Ec. However, in reality, the design window is quite small. If the step between the first and second diffusion is too small in depth, the curvature effect of the first diffusion edge becomes severe, resulting in electrical field E1 larger than Ec. And if the step is too big, it will not provide enough help to smooth out the second diffusion corner, resulting in electrical field E2 larger than Ec. It gives a very high requirement to the control of the diffusion process.
Yang et al. describe a method to form a step-like diffusion profile to suppress the edge breakdown in a single diffusion process step in U.S. Pat. No. 6,492,239. Before diffusion, a 0.3 um step in the InP is formed through wet chemical etching. However, in use the etch depth and sidewall curvature is very difficult to control by wet chemical etching. The diffused pn-junction profile strongly depends on a predetermined depth and curvature of etching. The alternative method is dry etching which can control the etch depth and the sidewall profile much better. But dry etching damages the InP surface severely and the surface condition/reconstruction is critical to achieve a repeatable diffusion profile.
Accordingly, a method of forming a smooth edge transition without a sharp curvature in the diffusion profile in a SAM-APD that will further lower electric fields in the edge of the active region remains highly desirable. An APD exhibiting improved gain uniformity across the active region of the device is equally desired.